This invention generally relates to apparatuses and methods of deploying and installing subsea equipment. More particularly, the present invention relates to wet parking, moving of, deployment, launching, and wet installation of subsea equipment.
Most subsea production systems are equipped with smaller components designed to be recovered and replaced using less expensive, non-invasive intervention techniques. These components include subsea control pods, specially designed valve and choke trim and actuators, pipeline maintenance and repair equipment, and fluid distribution modules. These components are typically designed to be placed and recovered by a free-swimming remotely operated vehicle (ROV) intervention system which is operated from a large support vessel. These subsea components usually require a soft landing on the manifold because of delicate components or interfaces.
Because of the need for a soft landing, the deployments system is usually mounted on a large, stable vessel such as a semi-submersible drilling rig or derrick barge. Smaller workboats are rarely used because their heave motion, even in modest seas, poses significant risk to the subsea equipment during loading, offloading, launching, landing, and recovery operations. Unfortunately, the high cost and questionable availability of large offshore vessels may prohibit their use.
As the need for new sources of oil and gas push operations into deeper water, such operations will increasingly require exacting placement of even larger and heavier subsea equipment and work packages 5,000 feet or more below the ocean""s surface.
The size and mass of the subsea equipment and the water depth absolutely precludes the use of divers. Similarly, the size and mass of many work packages precludes direct placement with ROVs. Buoyancy modules might assist ROV operations, but the mass of the work packages and the size of their required buoyancy may nevertheless preclude primary positioning operations with ROVs.
Directly lowering the subsea work package from a surface vessel on cables or other lines is well suited to accommodate the size and mass of large work packages. However, normal sea conditions subject the vessel to heave, thereby causing the vessel to fall and rise with the passing waves. Absent an effective active heave compensation system, the vessel""s motion is transmitted directly through the line to the subsea work package. This uncontrolled vertical motion proves unsatisfactory for many applications and has prevented final efforts by ROVs to guide and land the subsea work packages so presented.
Attempts have been made to dynamically compensate for the heave at the line, either by driving hydraulic rams or by driving a winch as necessary to take in or pay out line to maintain the subsea work package substantially stationary despite movement of the vessel. However, such systems are expensive, complex, subject to substantial maintenance requirements, and require delicate balance to operate effectively. Moreover, analysis has shown that deeper depths and heavier loads make these approaches to heave compensation ineffective. As is the case with smaller components, the alternative has been to avoid heave compensation systems and use semi-submersible drilling rigs or derrick barges for deployment of larger components. For example, the traditional way of deploying subsea trees and other hardware has been to use drill pipe deployed through the rig moonpool. This method ensures good uptime as heave motions are kept to a minimum on the very stable rig platform while package motions are not amplified dynamically due to the high stiffness of the drill pipe. On the other hand, the cost for using these large, stable vessels is extremely high for activities other than drilling and completing wells.
Accordingly, there remains a substantial need for a solution to the problem of placing heavy yet delicate subsea work packages in deepwater that is simple, straightforward, less costly, and otherwise suitable for real application in the offshore working environment.
The present invention is directed to apparatuses and methods of deploying and installing subsea equipment.
In one embodiment, the apparatus comprises a pendant line connecting the subsea equipment to a subsurface buoy; a deployment line having a catenary loop below the subsurface buoy, the deployment line being supported by the subsurface buoy on one end and connected to a surface vessel on the other end, the subsea equipment, subsurface buoy, pendant line, and deployment line cooperating to establish a natural frequency for the suspended subsea equipment which is materially different from the average wave frequency acting on the surface vessel; and a parking pile partially embedded in the sea floor, on which the subsea equipment may be parked.
In another embodiment, a method for positioning a subsea work package at a desired deepwater offshore location is described. The method includes launching a parking pile from a transport vessel; lowering the pile to the sea floor with a hoisting line; and then releasing the pile from the hoisting line such that the pile partially embeds itself into the sea floor. The method next includes launching the subsea work package from a transport vessel; lowering the subsea work package to the sea floor with a combination of wire, chain, clump weights, subsurface buoys, and synthetic line; and parking the subsea work package on the partially embedded pile. The parked subsea work package can then be moved to an operating location when desired.
The present invention also includes a method for positioning a subsea work package at a desired deepwater offshore location where the subsea work package is mounted to a parking pile. The combined parking pile and subsea package are launched from a transport vessel, lowered to the sea floor with a hoisting line; and then released from the hoisting line such that the pile partially embeds itself into the sea floor. If desired, a protective frame to surround the mounted subsea work package can be provided. Alternatively, a launching frame for launching the parking pile and subsea package from the transport vessel can be provided. Once parked, the subsea work package can be moved to a distant operating location.
The foregoing summary has outlined rather broadly the features and technical advantages of the present invention so that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter, which form the subject of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed might be readily used as a basis for modifying or designing other apparatuses and methods for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth and claimed herein.